How to add simple tariff sensor to my water meter

Configuration
1

I have following sensor and utility made for my water meter.
How can i add simple tariff to the utilitymeter?

My water cost 5.1€ / m^3

mqtt:
 sensor:
    -  name: "Water meter value"
       state_topic: "water/main/value"
       unit_of_measurement: m^3
       icon: "mdi:water"